The present invention relates generally to radar cross-section (RCS) measurement and antenna pattern ranges. More specifically, the present invention relates to low RCS measurement ranges employing a geometrically shaped measurement chamber to remove all but direct path reflections from the target of interest. Moreover, the present invention pertains to high performance anechoic chambers for antenna pattern measurement.
A radar system tracks a target in response to an echo, i.e., a reflected portion of the incident radar signal, from the target of interest. Therefore, it is critical to the design and operation of radar systems to be able to quantify, or otherwise describe this echo, particularly in terms of target characteristics, e.g., size, shape and/or orientation. One such characteristic is radar cross-section (RCS), which is the projected area of a metal sphere returning the same echo signal as the target of interest, assuming the metal sphere is substituted for the target of interest. Unlike the echo signal from a sphere, which is orientation independent, the echo signal, and thus the RCS, varies as a function of orientation of the target of interest. This variation can be very rapid, especially when the target of interest is many wavelengths in size.
RCS values of simple bodies can be computed exactly by solution of a wave equation defined in a coordinate system for which a constant coordinate coincides with the surface of the body. However, there is no known tactical target of interest which fits these solutions. The practical engineer cannot rely on predictions and calculations; the engineer must eventually measure the echo characteristics of the target of interest. This measurement can be performed on a full scale target of interest on an outdoor test range or on scale models to the target of interest in a measurement chamber. Current state of the art ranges include "compact ranges" which use a collimating reflector system to achieve the desired electromagnetic field distribution in the measurement zone (target area), i.e., to simulate a wide separation between the radar source and the target of interest.
Typical RCS chambers are rectangular rooms covered with Radar Absorber Materials (RAM). For a given target support system and antenna/radar system, the chamber performance is limited by the chamber size and shape and by absorber material employed. Cost limitations usually drive both the chamber size and the quantity and quality of the RAM installed in the chamber. The measurement capability in RCS chambers is limited by several factors, including:
(a) the chamber size and shape; PA1 (b) the type and amount of RAM applied to the chamber walls; PA1 (c) the target support system; and PA1 (d) the antenna radar system.
The room, i.e., measurement chamber itself, is often the limiting factor in RCS measurement chambers. Radar reflections or echo signals are generated by scattering from the target of interest. Echo signals which are not direct path may be either out of phase with the direct path echo signal or arrive at the radar at a later time than the direct path echo signal; thus, in either case, the non-direct path signals contaminate the RCS measurement. The conventional method of "quieting" the radar reflections from the room itself is by treating the chamber walls with large pyramidal RAM up to six feet deep that attenuates the incident microwave energy. In addition, radar range (time) "gating" (either true short pulse or synthetic short pulse) may be used to remove most radar reflected signals that arrive at the radar at a time other than the desired return from the target. However, since RAM provides only limited attenuation and since gating cannot provide echo signal cancellation to completely eliminate unknown short bounce interaction, and since range (time) gating cannot completely eliminate unwanted echo signals, the RCS measurement is usually contaminated by spurious echo signals. In other words, some diffuse returns from the absorber as well as some chamber wall returns will arrive at the radar at approximately the same time as the desired target return and cannot be gated out. These returns establish the background levels of the chamber. Since the target should be at least 10 decibels (dB) above the background level, this background level also establishes the limit on the lowest RCS target which can be measured using a given range.
Attempts have been made over the years to reduce the spurious scattered signals which contaminate RCS measurement. In particular, various proposals to improve or optimize the measurement chamber itself have been developed. For example, U.S. Pat. No. 4,507,660 disclosed an anechoic chamber which utilizes an expanded central area resembling two flared horns with their rims joined together and the vertexes of the anechoic chamber forming source and receiver portions. The purpose of this arrangement is to reduce the number of direct reflections to the target through a (structure) "dual-flared horn" geometry. A modification of this arrangement is disclosed in U.S. Pat. No. 5,631,661, which describes a modified dual flared horn having an internal diffraction edge absorber. An alternative structure is presented in U.S. Pat. No. 3,806,943, wherein a cylindrical chamber with conical end sections is disclosed. Other chamber configurations are disclosed in U.S. Pat. Nos. 3,308463, 3,100,870, 3,113,271 and 3,120,641.
Additionally, other chamber configurations are known. U.S. Pat. No. 4,931,798 discloses an elliptical anechoic chamber having RAM disposed at one to the two focal points of the ellipse. As shown in FIG. 1, the electromagnetic anechoic chamber includes an ellipsoidal metal shielding wall 10, which has an inner surface 12 defining a closed space 13. The inner surface 12 has first and second focus points 14 and 16 on a major axis 18 in the closed space 13. During operation, an electromagnetic wave is emitted from the first focus point 14 as an emitted wave, which is then reflected at the inner surface 12 of the shielding wall 10 towards the second focus point 16 as a scattered wave. Since the scattered wave is focused on the second focus point 16, the scattered wave can be absorbed by RAM disposed at the second focus point 16.
More specifically, as illustrated in FIG. 2, the electromagnetic anechoic chamber is configured for use in measurement of properties of one of receiving and transmitting antennas 22 and 24 which are placed in the closed space 13. The transmitting antenna 24 is located at the first focus point 14 and emits an electromagnetic wave as an emitted wave. The receiving antenna 22 is located between the first and the second focus points 14 and 16. The electromagnetic anechoic chamber further comprises absorption assembly 26 which is placed in the closed space 13. The absorption assembly 26 comprises an absorption member 28 and a supporting member 32 which supports the absorption member 28 at the second focus point 16. The absorption member 28 is made of material for absorbing the electromagnetic wave. In operation, the emitted wave travels in the closed space 13 and reaches be the shielding wall 10, where it is reflected by the inner surface 12 of the wall 10 into the closed space 13 as the scattered wave. Theoretically, the scattered wave is directed to the absorption member 28 disposed at the second focus point 16. As a result, the scattered wave is assumed to be effectively absorbed by the absorption member 28 and, thus, is never again reflected by the inner surface. Therefore, the '798 patent assumes that no resonance of the emitted wave can be caused and the scattered wave never reaches the receiving antenna 22.
It will also be appreciated that the emitted wave has a direct wave component which directly reaches the receiving antenna 22 without being reflected at the inner surface 12 of the shielding wall 10. In other words, the receiving antenna 22 receives only the direct wave. Therefore, it is theoretically possible to exactly measure the reception or transmission properties of one of the receiving and the transmitting antennas 22 and 24.
From the discussion above, it will be appreciated that the majority of the anechoic chambers mentioned above rely on absorbing materials, e.g., RAM, to prevent scattered waves generated either by scattering from the target of interest or emitted waves interacting with structures in the anechoic chamber from reaching the receiver. It will also be noted that RAM is a relatively large component of the overall cost of the anechoic chamber. Moreover, an anechoic chamber employing RAM has an associated maintenance cost, since RAM is subject to degradation over long periods of time. Moreover, frequent access to the measurement chamber by personnel for target change out and the like increases the degradation rate of the RAM. It will also be noted that an increase in the degree of absorbency provided by any particular RAM will generally be indicative of a non-linear cost increase for that RAM.
What is needed is a RCS measurement facility which emulates a free space measurement in that the only energy which arrives at the radar receiver is the direct backscatter from the target under test. It would also be highly beneficial if the RCS measurement facility or chamber were substantially devoid of RAM, thereby permitting fabrication of a more robust measurement chamber. In addition, what is needed is a measurement chamber which can be fabricated at a substantial cost savings, both in terms of construction and life cycle costs.